Currently you won't be able to get this knowledge, if you are playing with an old character (before release of the archer and thus before the release of the new storyquests in the early game). I just got it with my archer i created right after its release.

This also applies for:
AL RHUNDI, THE MORTAL https://bdocodex.com/cn/theme/5917/
THE ONLY THING THAT MATTERS https://bdocodex.com/cn/theme/5912/
A SPINELESS STRATEGY https://bdocodex.com/cn/theme/5914/
THE MAD LORD https://bdocodex.com/cn/theme/5916/
OZE'S WILL https://bdocodex.com/cn/theme/5915/
THE SHADOW KNIGHTS, FRATERNITY OF LIGHT , AND THE SECRET GUARDS https://bdocodex.com/cn/theme/5921/
THE FALL OF THE KNIGHTS OF DAWN https://bdocodex.com/cn/theme/5920/
THE COMING OF BELMORN AND THE KNIGHTS OF DAWN https://bdocodex.com/cn/theme/5919/
THE ORIGINS OF THE SHADOW KNIGHTS https://bdocodex.com/cn/theme/5918/
WAILING GRAVESTONE https://bdocodex.com/cn/theme/6333/
DARK RED CRYSTAL https://bdocodex.com/cn/theme/6332/
AN ETERNAL FLAME https://bdocodex.com/cn/quest/4540/1/
WHAT HE SAW AT THE NIGHT OF THE INCIDENT https://bdocodex.com/cn/theme/6273/
AN INCIDENT IN OMAR LAVA CAVE https://bdocodex.com/cn/theme/6272/
credits:OOWyvernOO

ok, how it get now, after new main quest update?